L3 fractures and faults

brittle deformation

permanent- non recoverable- change due to growth of fractures and/or sliding on pre-existing fractures

• forms when bending exceeds critical threshold

extension

• shows extension (movement) perpendicular to fracture walls

  • mode 1

shear

shows movement parallel to fracture walls- displacement is minimal

• sliding (mode 2) and tearing (mode 3)

joint sets

have similar orientation and morphology

can be systematic or non-systematic

• has a repeating pattern

joint system

two or more joint sets

orthogonal joint set

form in a rock with previous fractures

polygonal joint set

form in a rock without previous fractures

regular set + irregular, non systematic joint set

conjugate sets

2 joint sets formed at the same time

joints

a type of extensional fracture that shows a small wall-perpendicular opening with no or little shear offset.

systematic joints

joints that are straight, parallel and repeat themselves with more or less constant spacing

non-systematic joints

joints that have irregular geometry, orientation and spacing do not define sets.

joint tip interaction

plumose structures

characterized by feather-like markings that radiate from a point of origin on joint surfaces, indicating the direction of joint propagation.

fracturing is most common in …

the upper crust

columnar joints

• when lava cools and contracts, leading to the formation of fractures

as layer thickness increases…

spacing of joints increases

measuring joint density

1. average spacing of joints in a set

2. number of joints in a given area

3. total length of joints in a given area

4. surface area of all joints in a given volume

classification of faults are based on

1. orientation of the fault surface (strike/dip)

2. nature of the displacement or offset across the fault

also

3. character of the fault surface

fault

any surface or narrow zone with visible shear displacement along the zone.

• in the upper crust

• can be a discrete fracture or discontinuity that accounts for all of the displacement

fault damage zone

• the volume of brittlely deformed wall-rock, that is higher than the background level, that surrounds the fault core

• joints and microfaults very abundant

joints and microfaults

are often very abundant in damage zones

fault breccia

an unconsolidated fault rock consisting of less than 70% matrix

fault gauge

Fine-grained, clay-like, crushed rock Davis "like talcum powder with dispersed hard grains..."

Typically forms when wall rocks are relatively weak and incompetent

cataclastic rocks

less than 30 vol % are present in the fine grained matrix

• significant grain size reduction due to strain

significant grain size reduction due to strain (grain size >0.1 mm < 10 mm …can be extreme

• can accompany mylonitization

• looks like fault gauge but is cohesive

damage zone thickness increases …

as fault displacement increases

slickenlines

generally straight fine-scale shallow lines on fault surface which record direction of slip

slickenside

Smooth surface on fault caused by frictional polishing during movement (or due to neomineral surface coatings) - not present on many fault surfaces due to non-formation or erosion

Pseudotachylite

Black, extremely fine-grained dike-like features…seem to intrude the country rocks

• can actually be glassy

• represents a small volume of melt generated by frictional heating during a seismic event (earthquake)

lineaments

• linear topographic features of regional extent that are believed to reflect crustal structure

• straight or curved

• more common near recent faults, but occur with old ones as well

fault scarps

• vertical displacements produced by faults at the surface (typically several feet)

• usually the inclination and height are quickly reduced by erosion…

• can provide a tool for timing the relative age of faults

triangular facets

form when large fault scarps are dissected by erosion

geophysics

important tool for recognizing faults

shows breaks in the physical properties in the subsurface

a fault has maximum displacement…

near the center

a fault has the least displacement …

near the tip point

fault trace

the intersection between the fault surface and an arbitrary surface

tip points

the endpoints of the fault trace

tip line

the zero displacement line that outlines a fault

process zone

invisible on seismic data

• area ahead of the tip where the rock is '“processed” prior to fault propagation

fault tip

invisible on seismic data

horst

normal faults dipping away from each other create an upthrown block

graben

normal faults dipping toward each other create a downthrown block

half graben

main fault with antithetic (dips toward main fault) and synthetic (dips in same direction as main fault) faults

relay ramps

form from overlapping faults

footwall

block sitting below the fault surface

hanging wall

block sitting above the fault surface

normal fault/ normal-slip fault

hanging wall moves down with respect to footwall

• dip slip

• extensional faults

• ideally 60 degrees

thrust fault/ thrust-slip fault

hanging wall moves up

• compressional fault

• less than 45 degrees, usually 30 degrees

strike slip fault

horizontal translation or movement

• left handed or right handed

left handed (sinistral) strike slip fault

right handed (dextral) strike slip fault

dip slip

translation directly up or down fault surface

• extensional or compressional

low angle normal fault

• called detachment fault

• less than 45 degree dip

reverse-slip fault

hanging wall moves up

• compressional fault

• more than 45 degree dip

oblique-slip fault

• inclined

• displacement between strike-slip and dip-slip movement

• name using main component of movements eg. sinistral-normal or sinistral-reverse

rotational (scissor) fault

changes both magnitude and sense of slip along strike

slip

actual relative displacement

• the displacement vector between two points that were adjacent before faulting

net slip

Common terminology for slip emphasizes the fact we often don’t know the actual path blocks took to their present position

• can be broken down into strike slip and dip slip components

  • can be further divided into: vertical slip, horizontal dip slip, horizontal slip

components to completely describe slip

1. orientation of the slip line on the fault plane (fault = strike/dip)

• rake in the fault plane- angle between strike of FP and slip line

• trend and plunge of the slip line

2. magnitude of displacement (distance)

3. relative displacement - eg footwall up to the west

how do we measure slip?

must find two points, called “piercing points” that were adjacent before faulting

1. points

2. lines

points

• quite rare in geology

• ex, fossil cut by a fault

lines

• are very common

• intersect planes at a point

• eg, dike and bedding, unconformity and bedding

• facies changes

  • edges of sand lenses

  • lines of constant thickness

drag folds

distortion of bedding (or any pre faulting structures such as foliation, fractures, etc.) from shearing of rock bodies past one another; forms some ideal relationships

• axes are ideally perpendicular to slip direction

normal fault drag fold

hanging wall dragged in syncline, footwall into anticline

• exception with listric (shallowing dip with depth) faults of rollover anticlines

strike-slip drag folds

beds curl into fault away from movement

separation

apparent relative displacement

• we usually observe this on faults not slip

• has no direct relation to slip

throw

vertical component of dip separation

heave

horizontal component of dip separation

left lateral fault map view

right-lateral fault map view

stratigraphic throw (stratigraphic separation)

thickness of strata between two points now adjacent along a fault

• measured in various planes (map view and x section)

true separation

separation perpendicular to trace of the plane of the fault

strike separation

component of separation parallel to the strike of the fault

dip separation

component of separation parallel to the dip of the fault

horizontal separation

separation in a specified horizontal direction

vertical separation

separation measured vertically b/w a plane and its extension from the other side of a fault

overlap

amount of along strike overlap of strata viewed across fault

gap

amount of along strike gap of strata viewed across fault

offset (normal separation)

amount of separation of strata measured across the fault perpendicular to strike of strata